Device and method for generating a differential voltage

ABSTRACT

A circuit and a method are described for generating differential voltages.

FIELD OF THE INVENTION

The present invention relates to a device and a method for generating a differential voltage.

BACKGROUND INFORMATION

To generate a differential voltage, a first terminal is connected to a supply voltage via a first switch and a first current limiting and a second terminal is connected to ground via a second switch and a second current limiting. Since the switches do not switch at exactly the same time and the currents swell to a differently strong extent at the terminals depending on the temperature and aging, common-mode interferences develop which interfere with the differential signal transfer in the further course due to asymmetric conductor guidance.

Common-mode interferences due to asymmetric signal generation should be avoided, so that the signals are less interfered with.

SUMMARY

The present invention provides the advantage that common-mode interferences due to asymmetric signal generation are prevented. Thus, the signals are less interfered with.

It is provided to generate a corresponding differential signal with the aid of a current from an electrically isolated energy source.

In the embodiment according to the present invention, a capacitor C₁ may be charged when switches S₃ and S₄ are closed, switches S₁ and S₂ being open at the same time. After charging the capacitor, switches S₃ and S₄ are opened. To generate a differential voltage, switches S₁ and S₂ are closed. Due to the electrical isolation, it is possible to open and close switches S₁ and S₂ with a time delay, without an undesirable common-mode signal resulting therefrom. The currents via switches S₁ and S₂ and thus at corresponding terminals L and H always have exactly the same absolute value.

Preferably, those capacitors should be used which may be implemented in silicon. To generate longer signals, a second parallel circuit according to the present invention is thus provided, it being possible to permanently achieve a desired level.

It is also possible to generate a negative level, as is implemented in a third specific embodiment.

Additional advantages and advantageous embodiments may be derived from the description of the figures and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit drawing of one exemplary embodiment of the circuit according to the present invention.

FIG. 2 shows a circuit drawing of a second exemplary embodiment of the circuit according to the present invention.

FIG. 3 shows a circuit drawing of a third exemplary embodiment of the circuit according to the present invention.

FIG. 4 shows a flow chart of one exemplary embodiment of the method according to the present invention.

FIG. 5 shows a flow chart of a second exemplary embodiment of the method according to the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a circuit for generating differential voltages. The differential voltage is measured at terminals H and L. A current limiting I_(lim), a switch S₁, a first capacitor C₁, and a switch S₂ are connected between terminals H and L.

First capacitor C₁ is provided to facilitate a current flow I₁ from terminal L to terminal H in the charged state. A current cannot flow from L to H until both switches S₁ and S₂ are closed. The currents via S₁ and S₂ and thus at terminals L and H have exactly the same absolute value, regardless of temperatures, aging, or the like. If, for example, a differential voltage of 2 volts is to be achieved and the cable impedance is 60 ohm, then the current is limited to 33 mA with the aid of current limiting I_(lim). It is also conceivable, however, that larger or smaller currents flow, so that the desired differential voltage may be achieved faster or have a higher degree of accuracy.

In this process, first capacitor C₁ is discharged.

In order to charge first capacitor C₁, two further switches S₃ and S₄ are provided. When switches S₃ and S₄ are closed, a potential V_(CC) is connected to GND via first capacitor C₁ and first capacitor C₁ is charged.

FIG. 2 shows a circuit for generating differential voltages. The design is similar to the design of the circuit, such as the one shown in FIG. 1, a second capacitor C₂ being connected in parallel to first capacitor C₁. Second capacitor C₂ is connected between the two switches S₅ and S₆.

If first capacitor C₁ is exhausted, although the differential voltage is to be continuously maintained, second capacitor C₂ is provided to facilitate the flow of current I₂ from L to H. Current I₂ flows from L to H, when both switches S₅ and S₆ are closed. During this time, first capacitor C₁ may be recharged, so that its energy is available as soon as second capacitor C₂ requires charging.

In order to charge second capacitor C₂, two further switches S₇ and S₈ are provided. When switches S₇ and S₈ are closed, a potential V_(CC) is connected to GND via second capacitor C₂ and second capacitor C₂ is charged.

As a result of the mutual support of capacitors C₁ and C₂, it is possible to permanently achieve a desired voltage level, while the capacitors may have smaller dimensions, since they must be able to store only a fraction of the energy necessary to transfer one or multiple bits.

If the supply is switched from first capacitor C₁ to second capacitor C₂, switches S₅ and S₆ are closed in addition to already closed switches S₁ and S₂ and then switches S₁ and S₂ are opened. Two diodes D₁ and D₂ are tasked with allowing switches S₁, S₂, S₅, and S₆ to be closed simultaneously, without a high compensating current being able to flow from one of capacitors C₁, C₂ to the respective other capacitor. As soon as switches S₁ and S₂ are open, first capacitor C₁ may be charged when switches S₃ and S₄ are closed.

FIG. 3 shows two circuits, such as the ones shown in FIG. 2, connected in series.

If negative differential voltage changes or negative differential voltages are supposed to be generated, switches S₁₁ through S₁₈ are actuated in the same manner as switches S₁ through S₈.

FIG. 4 shows a flow chart of the method according to the present invention for generating differential voltages.

At beginning 100, a first capacitor C₁ is charged. In a step 110, a switch S₁ and a switch S₂ are closed. Thereupon, a current I₁ flows from a terminal L to a terminal H. As a result, first capacitor C₁ is discharged. In [step] 120, switches S₃ and S₄ are closed. As a result, a potential V_(CC) is connected to GND via first capacitor C₁ and first capacitor C₁ is charged.

FIG. 5 shows a flow chart of the method according to the present invention for generating differential voltages.

At beginning 100, a first capacitor C₁ is charged. In step 110, switch S₁ and switch S₂ are closed. Thereupon, a current I₁ flows from a terminal L to a terminal H. As a result, first capacitor C₁ is discharged.

In [step] 120, switches S₅ and S₆ are closed. The two diodes D₁ and D₂ are tasked with allowing switches S₁, S₂, S₅, and S₆ to be closed simultaneously, without a high compensating current being able to flow from one of capacitors C₁, C₂ to the respective other capacitor. In [step] 130, switches S₁ and S₂ are opened. As soon as switches S₁ and S₂ are open, switches S₃ and S₄ are closed, so that first capacitor C₁ may be charged. 

What is claimed is:
 1. A circuit for generating a differential voltage, comprising: a first terminal; a second terminal; a first capacitor; a first switch; and a second switch.
 2. The circuit as recited in claim 1, wherein the first capacitor facilitates a first current flow from the second terminal to the first terminal in a charged state, when the first switch and the second switch are closed.
 3. The circuit as recited in claim 1, further comprising: a third switch; a fourth switch, wherein the first capacitor is charged when the third switch and the fourth switch are closed.
 4. The circuit as recited in claim 2, further comprising: a second capacitor for at least one of: adding a second current flow to the first current flow via the first capacitor in the charged state, and allowing the second current flow when the first current flow via the first capacitor stops.
 5. The circuit as recited in claim 4, further comprising: a fifth switch; and a sixth a switch, wherein the second capacitor facilitates the second current flow from the second terminal to the first terminal, when the fifth switch and the sixth switch are closed.
 6. The circuit as recited in claim 5, further comprising: a seventh switch; and an eighth switch, wherein the second capacitor is charged when the seventh switch and the eighth switch are closed.
 7. A method for generating a differential voltage with the aid of a circuit that includes a first terminal, a second terminal, a first capacitor, a first switch, and a second switch, wherein a first current flows from the second terminal to the first terminal, when the first capacitor is charged, and the first switch and the second switch are closed.
 8. The method as recited in claim 7, wherein the first capacitor is charged, when third and fourth switches are closed.
 9. The method as recited in claim 8, wherein a second current flows from the second terminal to the first terminal, when the first current flow via the first capacitor stops and when a second capacitor is charged, and a fifth switch and a sixth switch are closed.
 10. The method as recited in claim 9, wherein the second capacitor is charged, when seventh and eighth switches are closed. 